Radiation and carcinogen-induced DNA damage and repair in human cancer-prone genetic diseases is being studied. Attention is presently focused on xeroderma pigmentosum (XP), a disease with ultraviolet sensitivity, and ataxia telangiectasia (AT), a disease which recently has been found to exhibit X-ray sensitivity. A detailed comparison of the clinical features of these diseases was made and demonstrated remarkable similarities, particularly in the neurological abnormalities in both diseases. Cell fusion studies to demonstrate possible genetic heterogeneity of DNA repair defects among AT strains have begun. Cell fusion studies have also begun to attempt to detect persons heterozygous for XP or AT. Correlation of clinical features of XP and AT with in vitro DNA repair defects may lead to further understanding of the link between DNA repair and cancer.